1. Field of the Invention
The present invention relates to a voltage-controlled oscillator (hereinafter called xe2x80x9cVCOxe2x80x9d) whose oscillation frequency can be controlled according to a control voltage.
2. Description of the Related Art
A conventional VCO has ring oscillators each comprised of a plurality of inverters and a transfer gate (hereinafter called xe2x80x9cTGxe2x80x9d) connected in ring form. An oscillating operation of the VCO is performed at a frequency corresponding to a loop delay time of each ring oscillator. The loop delay time corresponds to the sum of delay times of the inverters and a delay time of the TG. The delay time of each TG changes according to a control voltage supplied to a control terminal. Namely, when the control voltage is lowered, the TG increases in channel resistance to thereby make an increase in delay time, whereby an oscillation frequency is reduced. When the control voltage is raised in reverse, the TG is reduced in channel resistance to thereby make a decrease in delay time, whereby an oscillation frequency increases. It is thus possible to control the oscillation frequency over a relatively wide range.
However, the conventional VCO has the following problems.
One of them is that since the frequency to be fixed is already determined when, for example, the VCO is used to configure a phase-locked loop (hereinafter called xe2x80x9cPLLxe2x80x9d), a synchronous pull-in time at phase synchronization becomes long under the influence of a wide frequency control range that would be a characteristic of the conventional VCO.
Further, another is that due to insertion losses and gate capacitance of field-effect transistors (hereinafter called xe2x80x9cFETsxe2x80x9d) constituting each TG, a delay time increases and hence the upper limit of an oscillation frequency is restricted.
An object of the present invention is to provide a VCO which obtains a high oscillation frequency and is short in synchronous pull-in time when it is used in PLL.
In order to solve the above problems, the present invention provides a VCO comprising inverting amplifier circuits each comprised of an odd number of inverting amplifiers connected in tandem, feedback transistors which are connected between the outputs and inputs of the inverting amplifier circuits and whose conducting states are controlled according to a control voltage supplied to control electrodes thereof, respectively, and feedback resistors each connected in parallel with the feedback transistor between the output and input of the inverting amplifier circuit.